Adrenoceptor Antagonists

It is also called β‐blockers drugs .Most common example of 阝-blockers that are following:-
1.Acebutolol
2.Atenolol
3.Betaxolol
4.Bucindolol
5.Carteolol
6.Carvedilol
7.Doxazosin
8.Esmolol
9.Labetalol
10.Medroxalol
11.Metoprolol
12.Nadolol
13.Phenoxybenzamine
14.Phentolamine
15.Pindolol
16.Prazosin
17.Propranolol
18.Terazosin
19.Timolol
20.Tolazoline
21.Trimazosin

ADRENOCEPTORS:-

                                     Drugs that produce responses by interacting with adrenoceptors are referred to as adrenoceptor agonists or adrenergic agonists. 

        Norepinephrine and isoproterenol are examples of such compounds. Agents that inhibit responses mediated by adrenoceptor activation are known as adrenoceptor antagonists, adrenergic antagonists, or adrenergic blocking agents. Prazosin and propranolol are examples of receptor-blocking drugs. 

         The pharmacology of the adrenoceptor antagonists is described in this chapter. Norepinephrine is released from the varicosities of the postganglionic sympathetic nerves during neural activity and interacts with the adrenoceptors of the effector organ, producing the characteristic response of the effector. 

       This occurs because norepinephrine has an affinity for the receptors and possesses intrinsic activity; that is, it has the capacity to activate the receptors. Circulating catecholamines and other directly acting adrenomimetic drugs also interact with these receptors. 

      The adrenergic blocking agents also have an affinity for the adrenoceptors. The antagonists, however, have only limited or no capacity to activate the receptors; that is,they have little or negligible intrinsic activity.The blocking drugs compete with adrenomimetic substances for access to the receptors.

     Thus,these agents reduce the effects produced by both sympathetic nerve stimulation and by exogenously administered adrenomimetics. This action forms the basis for their therapeutic and investigational use.

     Competition for receptors, hence receptor antagonism, is governed by the law of mass action; that is, the interaction between drug and receptor depends on the concentration of drug in the vicinity of the receptor and the number of receptors present. Because agonist and antagonist have an affinity for the same receptors, the two substances compete for binding to the receptors.

CLASSIFICATION OF BLOCKING DRUGS:-

   An α-receptor is one that mediates responses for which the adrenomimetic order of potency is epinephrine greater than or equal to norepinephrine greater than isoproterenol, and that is susceptible to blockade by phentolamine and phenoxybenzamine. It follows from this definition that phentolamine and phenoxybenzamine are called α-adrenoceptor antagonists or α-blocking agents. 

A β-receptor mediates responses for which the adrenomimetic order of potency is isoproterenol greater than epinephrine greater than or equal to norepinephrine, and this receptor is susceptible to blockade by propranolol.Propranolol is,therefore,called a β-adrenoceptor antagonist or β-blocking agent.

β -Receptor Subtypes :-

The two main types of β-receptors have been given the designations β1 and β2.Among the responses mediated by β1-receptors is cardiac stimulation, whereas β2 receptor stimulation mediates bronchodilation and relaxation of vascular and uterine smooth muscle. These findings are significant,since a number of both agonists and antagonists have some degree of selectivity for either β1- or β2-receptors. 

α -Receptor Subtypes:-

                                      There are differences between the receptors on nerves (presynaptic receptors) and those on effector cells. Furthermore, some α-agonists and antagonists exhibit selectivity for one of these receptor types.